1. Field of the Invention
The present invention relates to a high-density magneto-optical recording medium and a reproducing method for information recorded on the medium.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in information quantity gives rise to a desire for higher densities of the medium. While the higher densities may be realized by reducing the space of recorded marks, the recording and reproducing of the marks are limited by the size of a light beam (beam spot) on the medium. When the presence of only one recorded mark in the beam spot is set, an output waveform corresponding to "1" or "0" may be observed as a reproduction signal according to whether the recorded mark is present or absent in the beam spot.
However, when the presence of plural recorded marks in the beam spot is set by reducing the space of the recorded marks, no change in reproduction output occurs regardless of movement of the beam spot on the medium. Accordingly, the output waveform becomes linear and the presence or absence of the recorded mark in the beam spot cannot be identified. To reproduce such small recorded marks having a period smaller than the size of the beam spot, it is sufficient to reduce the beam spot to a small size. However, since the size of the beam spot is limited by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens, the beam spot cannot be sufficiently reduced to a small size.
There has recently been proposed a reproducing method using magnetically induced super resolution such that a recorded mark smaller in size than the beam spot can be reproduced by the use of an existing optical system. According to this method, the resolution of reproduction is improved by masking other marks during reproduction of one mark in the beam spot. Accordingly, a super resolution disk medium is required to have at least a mask layer or a reproducing layer for masking other marks so that only one mark may be reproduced during signal reproduction, in addition to a recording layer for recording marks. A magneto-optical recording medium using a perpendicular magnetization film as the reproducing layer is proposed in Japanese Patent Laid-open No. 3-88156. In the prior art described in this publication, however, an initializing magnetic field of about several kOe is required to initialize the reproducing layer. Accordingly, a recording apparatus cannot be made compact.
On the other hand, a magneto-optical recording medium using a magnetic film as the recording layer is proposed in Japanese Patent Laid-open Nos. 5-81717 and 5-342670. This magnetic film has an easy direction of magnetization in a plane at room temperature and has an easy direction of magnetization perpendicular to a film surface at a given temperature or higher. The principle of reproduction in this prior art will now be described in brief with reference to FIGS. 31A, 31B, and 31C. As shown in FIG. 31C, a magneto-optical disk 2 is formed by laminating a magnetic reproducing layer 6 and a magnetic recording layer 8 on a transparent substrate 4. The magnetic reproducing layer 6 has an easy direction of magnetization in a plane at room temperature. However, when the medium is heated to a given temperature or higher by applying a reproducing power, the easy direction of magnetization is changed to a perpendicular direction. The magnetic recording layer 8 is a perpendicular magnetization film. Reference numeral 10 denotes a light beam.
The intensity distribution of the light beam is a Gaussian distribution as shown in FIG. 31A. Accordingly, when the disk is at rest, the temperature distribution on the disk is also a similar distribution such that the central portion is higher in temperature than the peripheral portion. In operation, however, the disk 2 is rotated in the direction of arrow R shown in FIG. 31C during reproduction. Accordingly, the temperature distribution on the disk in rotation becomes a distribution as shown in FIG. 31B so that a high-temperature area in the beam spot is shifted to the forward direction of rotation of the disk. Owing to such a temperature distribution during reproduction, the easy direction of magnetization of the magnetic reproducing layer 6 becomes an in-plane direction in a low-temperature area in the beam spot. Therefore, the Kerr rotation angle of reflected light becomes almost zero in the low-temperature area. In the high-temperature area, the easy direction of magnetization of the magnetic reproducing layer 6 is changed from an in-plane direction to an perpendicular direction.
The perpendicular magnetization of the magnetic reproducing layer 6 at this time is bonded to the magnetization of the magnetic recording layer 8 by an exchange force, and the direction of magnetization of the reproducing layer 6 is made identical with the direction of magnetization of the recording layer 8, thereby allowing the magnetization recorded in the recording layer 8 to be transferred to the reproducing layer 6. The area size of such transfer can be changed by varying a reproducing laser beam power. In this manner, the size of the masking reproducing layer is controlled so as to allow the reproduction of only one recorded mark, thereby obtaining the same effect as that in the case of substantially reducing the area of the beam spot.
As mentioned above, the intensity distribution of the laser beam 10 directed onto the disk 2 is a Gaussian distribution, and the disk 2 is rotated in the direction of arrow R. As a result, a low-temperature area 10a and a high-temperature area 10b are formed on the reproducing layer 6 (see FIG. 32). The high-temperature area 10b is shifted to the forward direction of rotation of the disk 2 with respect to the laser beam 10. In the prior art disclosed in Japanese Patent Laid-open No. 5-81717, however, the in-plane magnetization of the reproducing layer 6 in the low-temperature area 10a in the beam spot is bonded to the perpendicular magnetization of the recording layer 8, causing inclination of the in-plane magnetization to generate a perpendicular component as shown in FIG. 32.
As a result, the masking effect is reduced and a mark recorded on the recording layer adjacent to a mark to be reproduced cannot be perfectly masked. Accordingly, the magnetization of the recording layer in the low-temperature area is also transferred to the reproducing layer, so that individual marks cannot be identified because of interference causing a reduction in reproduction output.
Japanese Patent Laid-open No. 5-342670 mentioned above discloses a magneto-optical recording medium having a magnetic intermediate layer interposed between a magnetic reproducing layer and a magnetic recording layer. The magnetic intermediate layer is provided to prevent the possibility that when the exchange bonding force between the recording layer and the reproducing layer is too strong, the magnetization direction of the reproducing layer becomes perpendicular in an area where the laser beam is not directed, thereby reducing the masking effect of the reproducing layer. The magnetic intermediate layer described in this publication is considered from its composition to have a Curie point lower than a temperature of the medium to be heated by the reproducing laser beam. While the operation of the magnetic intermediate layer is not described in detail in this publication, it may be considered as follows:
When the magnetic intermediate layer is heated to temperatures higher than its Curie temperature, the magnetization of the intermediate layer disappears. At this time, in the low-temperature area in the beam spot, a stable in-plane mask is formed in the reproducing layer, while in the high-temperature area, the magnetization of the recording layer is transferred to the reproducing layer by a magnetostatic bond. Accordingly, information recorded on the medium in the high-temperature area can be read out. However, the transfer of the magnetization by a magnetostatic bonding force is weaker than the transfer of the magnetization by an exchange bonding force. Thus, the medium having the magnetic intermediate layer described in this publication is not satisfactory in transfer characteristic of the magnetization in the high-temperature area in the beam spot. In addition, the magnetostatic bonding force between the recording layer and the reproducing layer is absorbed by the magnetic intermediate layer, thus further hindering the transfer characteristic of the magnetization.